- Email: [email protected]
THE FAZAKERLEY RESPIRATOR
Brit. J Anaesth. (1956), 28, 176
THE FAZAKERLEY RESPIRATOR BY J. R. ESPLEN
Aintree Hospital, Liverpool Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
lingering and exceptionally distressing terminal illness. One of the disadvantages of positive pressure ventilation is that the method is not only incapable of assisting the patient IT is a daily event in anaesthesia, and not in exhaling but also capable of causing uncommon in other spheres of medical exhalation to be prolonged: for it is practice, to meet with the patient who is practicable only when the diaphragm and in need of relief from the burden of in- the muscles of the chest wall are atonic; haling; and one means of relieving such a and when these muscles are atonic the patient of his burden is the application of lungs are apt to deflate slowly, particupositive pressure ventilation.* . larly if they are fibrotic or oedematous or The safe application of positive pres- if the chest wall has lost its natural resilisure ventilation is, however, no easy ence. Now if exhalation is unduly promatter: for much time, thought, and longed the patient may still be exhaling at energy, must be expended if the dis- the time the inflation pressure is applied; advantages of the method are to be sur- and in that event exhalation will be cut mounted. If they cannot be surmounted— short, and the resting volume of the lungs and the problems they give rise to increase will increase. in difficulty as the period of ventilation This increase can produce complications lengthens—complications develop, and, in in the circulatory, respiratory, and muscuthe long term medical case, it is then very soon apparent that the resort to positive lar systems. It may raise the mean intrapressure ventilation has failed: that it has pleural pressure and so impede the venous resulted in the conversion of what should return to the heart. It will bring about an have been a rapidly fatal condition into a increase in the volume of the deadspace air and so impair the efficiency of ventilaMoreover, by holding the chest * In this paper the terms " positive " and " negative " tion. are used to indicate whether a pressure should be partly expanded in the intervals between added to or subtracted from the atmospheric pressure in order to obtain its absolute value. They are also inflations, it will prevent the diaphragm used qualitatively, to indicate how a pressure with which one is dealing ranks in relation to the atmos- and the muscles of the chest wall from pheric pressure—a use which it is thought is justi- regaining their normal resting lengths, and fiable on the grounds that the absolute value of a positive pressure must always be greater, and the may thereby jeopardize the functional reabsolute value of a negative pressure less, than the covery of the internal intercostal muscles; the atmospheric value. 176 INTRODUCTION " In breathing there are two kinds of blessings: inhaling the air and exhaling it; the former is oppressive, the latter refreshing, so strangely is Life mingled. Thank God when He lays a burden on thee and thank Him when He takes it off."—GOETHE.
THE FAZAKERLEY RESPIRATOR
not infrequently such as would justify its being described as " blown ". That the deformity appears first in the upper and anterior parts of the chest is probably because there the internal thrust of the intrathoracic pressure is assisted by the external pull of the accessory muscles of respiration. It always heralds the approach of a rapid deterioration in the effectiveness of the pulmonary exchange, and it should not, of course, be permitted to develop—but how it is to be prevented from developing is another question, and one that is likely to remain unanswered until the orthopaedic implications of artificial ventilation are better understood than they are today. It is not improbable, however, that the structural changes produced by ventilation vary as the mean value of the ventilating pressure varies, and that they would be reduced to nil if the mean value of the ventilating pressure could be reduced to zero. But the mean value of the pressure cannot be reduced to zero when positive pressures are used alone: it can be so reduced only when two pressures of opposite sign are at work—when, for example, a positive inflation pressure alternates with a negative expiratory pressure. To put the matter in another way; an expansion strain requires for its correction a compression strain; and a negative expiratory pressure will create a compression strain by causing the chest to be compressed by the atmospheric pressure in the intervals between inflations. Another means of limiting the structural changes produced by ventilation would be to limit the expansion of the chest by the application of some form of support or " splint" to the chest wall.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
for these muscles will be held under tension all the time that the chest is held expanded—and muscle which has been stretched whilst in the paralysed state either takes a long time to recover its tone or fails to recover it at all. Even the relatively short applications of positive pressure ventilation used in anaesthesia may, by their effect of continually relaxing the external intercostals and continually stretching the internal intercostals, result in the former muscles recovering their tone before the latter; a circumstance that would interfere with the concerted action of these muscles in the immediate postoperative period and delay the return of the patient's full expulsive powers. Another disadvantage of positive pressure ventilation is that it subjects the chest to unidirectional stresses when the muscles are paralysed, be borne by the bone, cartilage and fibrous tissue, of the chest wall. These tissues are malleable or deformable; and it is because of that, and because the ventilating pressure, though a small enough force in itself, is applied to them at the rate of some 20,000 applications each 24 hours, that it is undesirable that the stress produced by the pressure should be unidirectional. Just how long the chest can withstand the stresses of positive or negative, pressure ventilation (for the latter too produces unidirectional stress) is uncertain. It has, however, been the writer's experience that a " ballooning out" of the upper and anterior parts of the chest wall is often to be detected by the end of the third or the fourth week of ventilation—and that the appearance of the chest by the end of the third or the fourth month of ventilation is
177
BRITISH JOURNAL OF ANAESTHESIA
178
THE RESPIRATOR
This, the Fazakerley respirator, provides positive inflation pressures of 10 to 30 cm H 2 O, and provides for the control of the frequency and the phases of the ventilatory cycle. And it can also be made FIG. 1 to develop a negative pressure in the airway during the exhaling or expiratory E.P.C., unit (2); a glass pressure bottle phase. (1); a manometer (6); a corrugated flexible For its motive power, the respirator de- connecting tube (4); and an air lead (5). pends on compressed air: the supply Of these components, the I.P.P. and the requirements being a flow of 20 to 30 E.P.C. units are those responsible, for the litres a minute and a supply pressure of production of the positive and the negative 100 to 360 mm Hg. In the absence of a pressures. They also provide the innovasuitable air supply, oxygen may be used, tory features of the respirator, and each but in the interests of economy the gas is unit will therefore be described in detail. usually diluted with air before it is deBefore they are described it must, howlivered to the machine, its admixture with ever, first be explained how the machine air being readily effected by means of an works as a whole, although a short exair-entraining device that will be des- planation is all that will be required, cribed later. because the general principles on which The principal parts of the respirator are the pressure-cycled type of respirator "arrowed" in figure 1. They are: an works are by now well known. Briefly intermittent positive pressure, or I.P.P., then, the Fazakerley respirator operates as unit (3); an expiratory pressure control, or follows:
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
To return for a moment to the source of some of these problems—namely, the prolongation of exhalation. That it is possible for exhalation to become dangerously prolonged follows from the fact that positive pressure ventilation plays no part in the production or in the control of pulmonary deflation; that it is a form of ventilation in which only one phase, usually less than one-half, of the ventilatory cycle is under the anaesthetist's full control. And it was to provide the anaesthetist with a mean of controlling both inflation and exhalation, and of assisting the patient in exhaling, that the respirator shown in figure 1 was designed.
179
THE FAZAKERLEY RESPIRATOR
The I.P.P. Unit (fig. 2). The air supplied to the respirator passes through the air-lead (A) to reach and enter the E.P.C. unit, which unit should, for the present, be regarded as a plain L-shaped connecting piece that has been fitted with a side-tube (S). From the E.P.C. unit the air is free to flow through the corrugated connecting tube (E), the distributor tube (D), the short flexible connection (F), and the cylinder (C), to an exhaust port (e). It is also free to flow into the glass pressure bottle by way of the mount (M). The port (e) is opened and closed by a mechanically operated valve (V), with the result that the air admitted to the apparatus is alternately able and unable to escape from the apparatus to the atmosphere; the period during which the port is open constituting the expiratory phase, and the period during which it is closed the inflation phase. c
CCA
FIG.
2
The Inflation Phase of the Ventilatory Cycle (figs. 2, 3, 4). When the exhaust port is closed, the air supplied to the respirator flows into a closed system. It therefore creates pressure. This pressure is applied to the bag in the bottle and, through a one-way valve (U), to a bellows (B). The bag in the bottle is compressed by the pressure and its contents are insufflated into the patient's lungs, which inflate. The bellows too inflate in response to the pressure—but not until the pressure in the system is great enough to overcome the tension of a spring (T).
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
Inside the glass pressure bottle is a onegallon rubber bag. This is connected with and employed as the rebreathing bag of the CO2 absorption apparatus that happens to be in use, when compression and expansion of the bag will produce inflation and deflation of the patient's lungs. The necessary compression and expansion of the bag is brought about by alternately raising and lowering the air pressure in the bottle: the I.P.P. unit producing a positive pressure in the bottle, and the E.P.C. unit a negative pressure (or an atmospheric pressure), turn about. By generating and applying these pressures the respirator acts as a pneumatic hand and serves as a mechanical means of ventilating the lungs.
BRITISH JOURNAL OF ANAESTHESIA
180
FIG.
3
This spring extends from the horizontal arm of a lever (L) to a hook located on the underside of the cylinder (C). Its action is to pull the lefthand end of the cylinder downwards against the bellows. The bellows therefore cannot expand until the expansion force of the air pressure inside them exceeds the compression force of the spring. When the bellows do inflate, they drive the lefthand end of the cylinder upwards, causing the cylinder to tilt on the pivot (X) by means of which it is mounted on the brackets (H). The slope of the cylinder is thus reversed, and the heavy ball (W) consequently rolls from the left- to the right-
FIG.
4
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
hand end of the angled perspex strip that forms the valve (V)—causing the valve to "see-saw" into a position which leaves open the port (e). With the opening of the exhaust port the inflation phase ends and the expiratory phase begins, but before the expiratory phase is described the means by which the inflation pressure and the length of the inflation phase are controlled must be explained. The Control of the Inflation Pressure ( % 2). The peak inflation pressure is the pressure present in the apparatus at the moment the exhaust port opens. It varies as the tension of the spring (T).
THE FAZAKERLEY RESPIRATOR
To increase the tension of the spring,
181
>-1000000^3-^-
~i
The Expiratory Phase and its Control (figs. 5 and 6). The opening of the exhaust port has the immediate effect of reversing the pressure gradient across the oneway valve (U), and the valve therefore closes. It also has the effect of dispersing the pressure in the bottle, so that the patient's lungs are free to deflate. The bellows, too, deflate, but to do so they must expel the air they contain through the rubber tube (G), since this tube is the only exit from the bellows. The tube is furnished with a clamp (CL), which the anaesthetist can open and close by turning the knob (R) anticlockwise and clockwise respectively, and the rate at which the bellows deflate depends on the extent to which the clamp obstructs its lumen. cNow the more rapidly that the bellows deflate, the more rapidly will the spring (T) draw the lefthand end of the cylinder downwards again, the sooner will the
FIG.
5
cylinder regain its original angle of slope, and the sooner will the ball (W) roll back to close the valve and bring the expiratory phase to an end. Contrariwise, the more slowly that the bellows deflate the longer will be the period that elapses before the expiratory phase ends. The knob (R) therefore controls the length of the expiratory phase of the cycle: to provide an expiratory pause, the anaesthetist has only to turn the knob clockwise until the bellows deflate more slowly than the lungs deflate. The Control of the Ventilatory Rate. The ventilatory rate (or the frequency of the mechanical cycle) is governed by
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
and so raise the inflation pressure, the anaesthetist turns the knob (P) clockwise. By doing so he screws a threaded rod to which the knob (P) is attached against the vertical arm of the lever (L), causing that arm of the lever to be pushed to the left and the horizontal arm of the lever to be depressed—when the spring will be further stretched and its tension increased. To lower the inflation pressure the anaesthetist turns the knob anticlockwise. The Control of the Length of the Inflation Phase. The length of the inflation phase varies as, and is controlled by varying, the rate at which air is supplied to the respirator.
BRITISH JOURNAL OF ANAESTHESIA
J82
6
the factors shown in the following equation: Rate =
60 Length of infla- + Length of expirtion phase in atory phase, inseconds eluding any pause, in seconds
cycles/min.
Anything that reduces the value of the denominator of the equation will increase the rate, and anything that increases the denominator of the equation will reduce the rate. In practice, the anaesthetist usually alters the rate by providing a slightly longer or shorter expiratory phase. The E.P.C. Unit (fig. 2). The upper one-third of the E.P.C. unit forms a small air chamber. This chamber
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
FIG.
has an inlet for the admission of air, the side tube (S), and two exits for its discharge: the annular space (O), and the bore of the jet (J). The first of these exits, the annular space, is a variable aperture, since clockwise rotation of the knob (K) will drive the cone-shaped point of the jet into the annular space, causing the space to close; whilst anticlockwise rotation of the knob will withdraw the jet, causing the space to enlarge. The second exit, the bore of the jet is a fixed aperture. Of the compressed air admitted to the air chamber, some will be discharged from the chamber by way of the bore of the jet valve, and some by way of the annular space; and the way in which the discharge of air is distributed between these two exits determines the pressure in the mount (M). For the air that is discharged from the chamber by the bore of the jet valve is injected into a venturi tube (N), and its entry into the venturi tube will tend to create a negative pressure in the mount; whereas the air that is discharged from the chamber by way of the annular space will enter the mount itself, under pressure—a circumstance that tends to create a positive pressure in the mount. The two tendencies vary in strength as the airflows by which they are produced vary in strength; and since the anaesthetist can, by manipulating the knob (K), increase the strength of either flow, it follows that he can accentuate either tendency, and so produce a positive or a negative pressure in the mount at will. And that he can produce a positive or a negative expiratory pressure—because the pressure produced in the mount will be
THE FAZAKERLEY RESPIRATOR
183
To produce a negative expiratory pressure. The anaesthetist turns the knob (K) clockwise in order to reduce the patency of the annular space and so increase its flow resistance. Once the flow resistance of the space exceeds the flow resistance of the bore of the jet valve, most of the air admitted to the air chamber will leave the chamber by the bore of the valve (i.e. by the easier route), with the result that more air will pass through the venturi tube than will pass into the mount. The dominant tendency will, therefore, be that produced by the venturi tube, and the pressure in the mount will become negative—and increasingly so as the knob continues to be turned. Once the space has been completely closed, no air at all can enter the mount and, the venturi effect being unopposed, the pressure in the mount may fall to as low as minus 10 cm EUO. To produce a positive expiratory pressure. The anaesthetist turns the knob (K) anticlockwise in order to increase the patency of the annular space, and so reduce its flow resistance. Once the flow resistance of the space has fallen below that of the bore of the jet valve, most of the air admitted to the air chamber will leave by way of the annular space. More
To produce atmospheric expiratory pressure. The anaesthetist turns the knob (K) one way or the other until the manometer registers atmospheric pressure during the expiratory phase. He will then in effect have matched the flow resistance of the annular space with that of the bore of the jet valve; and equal volumes of air will be passing into the venturi tube and the mount to produce equal but opposite effects. From the point of view of clinical usage, the E.P.C. unit may be said to function only during the expiratory phase of the cycle. The pressure created in the mount by the interaction of the two forces just described does, however, play a minor role during the inflation phase of the cycle. A negative pressure, for example, then acts as an anti-inflationary force; as is evident by the rise in the peak inflation pressure that occurs when a negative pressure is cancelled, and by the fall in the inflation pressure that occurs when the strength of a negative pressure is increased. Conversely, the provision of a positive expiratory pressure raises, and its cancellation lowers, the peak inflation pressure. These side effects to the use of the E.P.C. unit have one, apparently quite unimportant.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
propagated to the pressure bottle and re- air will therefore pass into the mount than produced in the rebreathing bag during will pass into the venturi tube and the the expiratory phase. During the inflation dominant tendency will be that produced phase, the small pressure produced in the by the expansion of air in the mount. The mount by the tendencies under discussion pressure in the mount accordingly becomes is overwhelmed to insignificance by the positive. Once the space is fully open, little or no air enters the venturi tube and, inflation pressure. How the anaesthetist adjusts the expira- the expansion effect of the air entering the tory pressure in practice is explained mount being unopposed, the pressure in the mount rises to around 5 cm H2O. below.
184
BRITISH JOURNAL OF ANAESTHESIA
capable of supplying up to 40 litres of airoxygen mixture a minute and of delivering the mixture, which contains 30 to 35 per cent oxygen, under a pressure of about 100 mm Hg. With the device in use 100 cubic feet of oxygen will run the respirator for from 6 to 8 hours, according to the MANOMETER; PRESSURE RELEASE VALVE; depth and the frequency of the ventilation provided. AIR-ENTRAINING DEVICE Here it would be as well to point out There remain to be described: the manthat the respirator is subject to the law ometer (fig. 2); the pressure release valve of supply and demand, and that the more (fig. 2 (RV)); and the air entraining work that it is called upon to perform, the device. more energy in the form of air does it The manometer is connected with the need to be provided with. The correct flow breathing circuit. Because of that, and be- is always that which provides satisfactory cause it is a water manometer, it serves not ventilation and the type of inflation phase only as a pressure indicator but as a safety (rapid or slow) that the anaesthetist valve as well, for the height and the depth favours. of the manometer tube will limit the positive and the negative pressures that the DIRECT INFLATION OF LUNGS machine can apply to the lungs. The lengths of the tubes joining the The pressure release valve (RV) con- E.P.C. unit with the I.P.P. unit make the sists of a rubber flap covering a hole in the respirator cumbersome and detract from bag mount (BM). The valve lifts to allow its appearance, but their lengths are a conexcess gas to pass from the breathing venience when it is necessary to ventilate circuit to the bottle, and thence to the a patient with air or with an air-oxygen atmosphere during the expiratory phase. mixture in an emergency. For the respiraIt prevents the rebreathing bag from over- tor to be used in this way, the E.P.C. unit distending during use. is detached from the pressure-bottle and The air-entraining device which is connected to the endotracheal or facemask not illustrated here saves oxygen when connection: and the long tubes enable the respirator is worked from a cylinder. this to be done with the minimum of delay. Designed to be connected with B.O.C. oxygen valve MS.55, or MS.56, by CONCLUSION AND APPARATUS FOR STUDIES OF VENTILATION a short length of stout-walled rubber tubing, and with the respirator by not In conclusion, this respirator was demore than 10 feet of ^-inch bore tubing, signed in the hope that the provision of a the device utilizes the injector principle to negative phase would enable some of the convert a small volume of oxygen at a high complications of positive pressure ventilapressure into a larger volume of air and tion to be avoided, but it is emphasized oxygen at a relatively low pressure. It is that as yet there is only the most tenuous result: that the difference between the maximum and the minimum pressure remains constant for any given setting of the inflation pressure control knob (P) no matter how the expiratory pressure control knob (K) may be set and reset.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
THE FAZAKERLEY RESPIRATOR
185
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
The lefthand bellows (M) are connected with the pressure bottle (P) of the respirator. The righthand bellows (N) are connected with the outlet (S), and with a tube (St) to which is attached a catheter whose tip lies near to the tracheal bifurcation. During the expiratory phase, the bellows (M) are emptied by the suction then present in the pressure bottle, or (if it is not intended to use a negative expiratory pressure) by means of weights placed on top of the bellows. As the bellows (M) empty, so the bellows (N) expand. And as the bellows expand they fill with air drawn from the trachea. During the inflation phase, the positive pressure in the pressure bottle expands the bellows (M) and as they expand they compress the bellows (N), driving the tracheal air contained in the latter towards and into the collecting bag attached at (S). Apparatus for the Continual Withdrawal Nonreturn valves incorporated in the bellows (N) maintain the necessary uniof Samples of the Tracheal Air directional flow. Figure 7 shows the respirator connected with a simple absorption system, in which the volume of the exhaled air may be measured by a meter placed in the expiratory side of the circuit, and in which the fresh anaesthetic gases are admitted at a point that precludes their passing through the meter during the expiratory phase. The rest of the apparatus is concerned with the withdrawal of air from the region of the tracheal bifurcation during, and only during, the expiratory phase, and with the delivery of the air withdrawn to a collecting bag attached at (S). (M) and (N) are two bellows linked in such a manner that the expansion of one bellows will result in the compression of the other. FIG. 7
evidence that the use of negative expiratory pressures is in any way protective. It should also be mentioned that the findings of a recent investigation suggest that there are occasions when the use of negative expiratory pressures is undesirable—during anaesthesia for intrathoracic procedures for example, when their use may lead to collapse of the contralateral lung, to underventilation with retention of CO2, and a dangerous reduction in the uptake of oxygen. In the course of this investigation it became necessary (i) to obtain serial samples of the air present in the trachea during expiration, and (ii) to record the mean ventilating pressure. To this end, two pieces of apparatus suitable for use in conjunction with the respirator were designed. Each is briefly described below.
BRITISH JOURNAL OF ANAESTHESIA
186
Tube leading to airway Open
FIG.
8
height (H) is the height of the water column that the pressure in the airway can sustain continuously: it is therefore the mean ventilating pressure, or a close approximation to that pressure. The water levels alter when the mean The Mean Pressure Manometer pressure alters, but they do not oscillate The following method of measuring the mean ventilating pressure is based on the (to any great extent) in response to fluctuafact that the mean or average pressure in tion of the inflation pressure. This is a system is the effective working pressure because the arrangement allows of the or the pressure continuously at work in displacement of a large volume of water over a period of minutes, but does not that system. allow of the displacement of more than a In figure 8 (A) and (B) are two bottles few cubic centimetres of water during the of large capacity joined together by a second or so that each pressure-phase lasts. syphon tube (T). Each bottle is partly ACKNOWLEDGMENTS filled with water. am glad to have this opportunity of thanking When bottle (A) is connected with the Dr.I H. J. V. Morton for his encouragement and helppatient's airway, the mean pressure will, ful suggestions; thanking Dr. A. B. Christie, Mr. Edwards, and Mr. Kenneth Waddington, for if it is greater than zero, work to displace Ronald the interest they have shown in the use of the respiraand of thanking all the other people who have, water from bottle (A) to bottle (B). At first tor; in one way and another, helped me to press on with the water level in bottle (B) rises steadily; the building of the machine and with the writing of paper. but very shortly it ceases to rise and re- thisI also acknowledge the technical assistance I have mains at a constant height above the received from Messrs Elliotts (Liverpool) Ltd., the manufacturers of the respirator. They could not have water level in bottle (A). The difference in been more co-operative. A tube leading to an infrared gas analyser is substituted for the collecting bag when a breath-by-breath analysis of the exhaled air is required.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
Water level (initial)
THE FAZAKERLEY RESPIRATOR BY J. R. ESPLEN
Aintree Hospital, Liverpool Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
lingering and exceptionally distressing terminal illness. One of the disadvantages of positive pressure ventilation is that the method is not only incapable of assisting the patient IT is a daily event in anaesthesia, and not in exhaling but also capable of causing uncommon in other spheres of medical exhalation to be prolonged: for it is practice, to meet with the patient who is practicable only when the diaphragm and in need of relief from the burden of in- the muscles of the chest wall are atonic; haling; and one means of relieving such a and when these muscles are atonic the patient of his burden is the application of lungs are apt to deflate slowly, particupositive pressure ventilation.* . larly if they are fibrotic or oedematous or The safe application of positive pres- if the chest wall has lost its natural resilisure ventilation is, however, no easy ence. Now if exhalation is unduly promatter: for much time, thought, and longed the patient may still be exhaling at energy, must be expended if the dis- the time the inflation pressure is applied; advantages of the method are to be sur- and in that event exhalation will be cut mounted. If they cannot be surmounted— short, and the resting volume of the lungs and the problems they give rise to increase will increase. in difficulty as the period of ventilation This increase can produce complications lengthens—complications develop, and, in in the circulatory, respiratory, and muscuthe long term medical case, it is then very soon apparent that the resort to positive lar systems. It may raise the mean intrapressure ventilation has failed: that it has pleural pressure and so impede the venous resulted in the conversion of what should return to the heart. It will bring about an have been a rapidly fatal condition into a increase in the volume of the deadspace air and so impair the efficiency of ventilaMoreover, by holding the chest * In this paper the terms " positive " and " negative " tion. are used to indicate whether a pressure should be partly expanded in the intervals between added to or subtracted from the atmospheric pressure in order to obtain its absolute value. They are also inflations, it will prevent the diaphragm used qualitatively, to indicate how a pressure with which one is dealing ranks in relation to the atmos- and the muscles of the chest wall from pheric pressure—a use which it is thought is justi- regaining their normal resting lengths, and fiable on the grounds that the absolute value of a positive pressure must always be greater, and the may thereby jeopardize the functional reabsolute value of a negative pressure less, than the covery of the internal intercostal muscles; the atmospheric value. 176 INTRODUCTION " In breathing there are two kinds of blessings: inhaling the air and exhaling it; the former is oppressive, the latter refreshing, so strangely is Life mingled. Thank God when He lays a burden on thee and thank Him when He takes it off."—GOETHE.
THE FAZAKERLEY RESPIRATOR
not infrequently such as would justify its being described as " blown ". That the deformity appears first in the upper and anterior parts of the chest is probably because there the internal thrust of the intrathoracic pressure is assisted by the external pull of the accessory muscles of respiration. It always heralds the approach of a rapid deterioration in the effectiveness of the pulmonary exchange, and it should not, of course, be permitted to develop—but how it is to be prevented from developing is another question, and one that is likely to remain unanswered until the orthopaedic implications of artificial ventilation are better understood than they are today. It is not improbable, however, that the structural changes produced by ventilation vary as the mean value of the ventilating pressure varies, and that they would be reduced to nil if the mean value of the ventilating pressure could be reduced to zero. But the mean value of the pressure cannot be reduced to zero when positive pressures are used alone: it can be so reduced only when two pressures of opposite sign are at work—when, for example, a positive inflation pressure alternates with a negative expiratory pressure. To put the matter in another way; an expansion strain requires for its correction a compression strain; and a negative expiratory pressure will create a compression strain by causing the chest to be compressed by the atmospheric pressure in the intervals between inflations. Another means of limiting the structural changes produced by ventilation would be to limit the expansion of the chest by the application of some form of support or " splint" to the chest wall.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
for these muscles will be held under tension all the time that the chest is held expanded—and muscle which has been stretched whilst in the paralysed state either takes a long time to recover its tone or fails to recover it at all. Even the relatively short applications of positive pressure ventilation used in anaesthesia may, by their effect of continually relaxing the external intercostals and continually stretching the internal intercostals, result in the former muscles recovering their tone before the latter; a circumstance that would interfere with the concerted action of these muscles in the immediate postoperative period and delay the return of the patient's full expulsive powers. Another disadvantage of positive pressure ventilation is that it subjects the chest to unidirectional stresses when the muscles are paralysed, be borne by the bone, cartilage and fibrous tissue, of the chest wall. These tissues are malleable or deformable; and it is because of that, and because the ventilating pressure, though a small enough force in itself, is applied to them at the rate of some 20,000 applications each 24 hours, that it is undesirable that the stress produced by the pressure should be unidirectional. Just how long the chest can withstand the stresses of positive or negative, pressure ventilation (for the latter too produces unidirectional stress) is uncertain. It has, however, been the writer's experience that a " ballooning out" of the upper and anterior parts of the chest wall is often to be detected by the end of the third or the fourth week of ventilation—and that the appearance of the chest by the end of the third or the fourth month of ventilation is
177
BRITISH JOURNAL OF ANAESTHESIA
178
THE RESPIRATOR
This, the Fazakerley respirator, provides positive inflation pressures of 10 to 30 cm H 2 O, and provides for the control of the frequency and the phases of the ventilatory cycle. And it can also be made FIG. 1 to develop a negative pressure in the airway during the exhaling or expiratory E.P.C., unit (2); a glass pressure bottle phase. (1); a manometer (6); a corrugated flexible For its motive power, the respirator de- connecting tube (4); and an air lead (5). pends on compressed air: the supply Of these components, the I.P.P. and the requirements being a flow of 20 to 30 E.P.C. units are those responsible, for the litres a minute and a supply pressure of production of the positive and the negative 100 to 360 mm Hg. In the absence of a pressures. They also provide the innovasuitable air supply, oxygen may be used, tory features of the respirator, and each but in the interests of economy the gas is unit will therefore be described in detail. usually diluted with air before it is deBefore they are described it must, howlivered to the machine, its admixture with ever, first be explained how the machine air being readily effected by means of an works as a whole, although a short exair-entraining device that will be des- planation is all that will be required, cribed later. because the general principles on which The principal parts of the respirator are the pressure-cycled type of respirator "arrowed" in figure 1. They are: an works are by now well known. Briefly intermittent positive pressure, or I.P.P., then, the Fazakerley respirator operates as unit (3); an expiratory pressure control, or follows:
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
To return for a moment to the source of some of these problems—namely, the prolongation of exhalation. That it is possible for exhalation to become dangerously prolonged follows from the fact that positive pressure ventilation plays no part in the production or in the control of pulmonary deflation; that it is a form of ventilation in which only one phase, usually less than one-half, of the ventilatory cycle is under the anaesthetist's full control. And it was to provide the anaesthetist with a mean of controlling both inflation and exhalation, and of assisting the patient in exhaling, that the respirator shown in figure 1 was designed.
179
THE FAZAKERLEY RESPIRATOR
The I.P.P. Unit (fig. 2). The air supplied to the respirator passes through the air-lead (A) to reach and enter the E.P.C. unit, which unit should, for the present, be regarded as a plain L-shaped connecting piece that has been fitted with a side-tube (S). From the E.P.C. unit the air is free to flow through the corrugated connecting tube (E), the distributor tube (D), the short flexible connection (F), and the cylinder (C), to an exhaust port (e). It is also free to flow into the glass pressure bottle by way of the mount (M). The port (e) is opened and closed by a mechanically operated valve (V), with the result that the air admitted to the apparatus is alternately able and unable to escape from the apparatus to the atmosphere; the period during which the port is open constituting the expiratory phase, and the period during which it is closed the inflation phase. c
CCA
FIG.
2
The Inflation Phase of the Ventilatory Cycle (figs. 2, 3, 4). When the exhaust port is closed, the air supplied to the respirator flows into a closed system. It therefore creates pressure. This pressure is applied to the bag in the bottle and, through a one-way valve (U), to a bellows (B). The bag in the bottle is compressed by the pressure and its contents are insufflated into the patient's lungs, which inflate. The bellows too inflate in response to the pressure—but not until the pressure in the system is great enough to overcome the tension of a spring (T).
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
Inside the glass pressure bottle is a onegallon rubber bag. This is connected with and employed as the rebreathing bag of the CO2 absorption apparatus that happens to be in use, when compression and expansion of the bag will produce inflation and deflation of the patient's lungs. The necessary compression and expansion of the bag is brought about by alternately raising and lowering the air pressure in the bottle: the I.P.P. unit producing a positive pressure in the bottle, and the E.P.C. unit a negative pressure (or an atmospheric pressure), turn about. By generating and applying these pressures the respirator acts as a pneumatic hand and serves as a mechanical means of ventilating the lungs.
BRITISH JOURNAL OF ANAESTHESIA
180
FIG.
3
This spring extends from the horizontal arm of a lever (L) to a hook located on the underside of the cylinder (C). Its action is to pull the lefthand end of the cylinder downwards against the bellows. The bellows therefore cannot expand until the expansion force of the air pressure inside them exceeds the compression force of the spring. When the bellows do inflate, they drive the lefthand end of the cylinder upwards, causing the cylinder to tilt on the pivot (X) by means of which it is mounted on the brackets (H). The slope of the cylinder is thus reversed, and the heavy ball (W) consequently rolls from the left- to the right-
FIG.
4
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
hand end of the angled perspex strip that forms the valve (V)—causing the valve to "see-saw" into a position which leaves open the port (e). With the opening of the exhaust port the inflation phase ends and the expiratory phase begins, but before the expiratory phase is described the means by which the inflation pressure and the length of the inflation phase are controlled must be explained. The Control of the Inflation Pressure ( % 2). The peak inflation pressure is the pressure present in the apparatus at the moment the exhaust port opens. It varies as the tension of the spring (T).
THE FAZAKERLEY RESPIRATOR
To increase the tension of the spring,
181
>-1000000^3-^-
~i
The Expiratory Phase and its Control (figs. 5 and 6). The opening of the exhaust port has the immediate effect of reversing the pressure gradient across the oneway valve (U), and the valve therefore closes. It also has the effect of dispersing the pressure in the bottle, so that the patient's lungs are free to deflate. The bellows, too, deflate, but to do so they must expel the air they contain through the rubber tube (G), since this tube is the only exit from the bellows. The tube is furnished with a clamp (CL), which the anaesthetist can open and close by turning the knob (R) anticlockwise and clockwise respectively, and the rate at which the bellows deflate depends on the extent to which the clamp obstructs its lumen. cNow the more rapidly that the bellows deflate, the more rapidly will the spring (T) draw the lefthand end of the cylinder downwards again, the sooner will the
FIG.
5
cylinder regain its original angle of slope, and the sooner will the ball (W) roll back to close the valve and bring the expiratory phase to an end. Contrariwise, the more slowly that the bellows deflate the longer will be the period that elapses before the expiratory phase ends. The knob (R) therefore controls the length of the expiratory phase of the cycle: to provide an expiratory pause, the anaesthetist has only to turn the knob clockwise until the bellows deflate more slowly than the lungs deflate. The Control of the Ventilatory Rate. The ventilatory rate (or the frequency of the mechanical cycle) is governed by
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
and so raise the inflation pressure, the anaesthetist turns the knob (P) clockwise. By doing so he screws a threaded rod to which the knob (P) is attached against the vertical arm of the lever (L), causing that arm of the lever to be pushed to the left and the horizontal arm of the lever to be depressed—when the spring will be further stretched and its tension increased. To lower the inflation pressure the anaesthetist turns the knob anticlockwise. The Control of the Length of the Inflation Phase. The length of the inflation phase varies as, and is controlled by varying, the rate at which air is supplied to the respirator.
BRITISH JOURNAL OF ANAESTHESIA
J82
6
the factors shown in the following equation: Rate =
60 Length of infla- + Length of expirtion phase in atory phase, inseconds eluding any pause, in seconds
cycles/min.
Anything that reduces the value of the denominator of the equation will increase the rate, and anything that increases the denominator of the equation will reduce the rate. In practice, the anaesthetist usually alters the rate by providing a slightly longer or shorter expiratory phase. The E.P.C. Unit (fig. 2). The upper one-third of the E.P.C. unit forms a small air chamber. This chamber
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
FIG.
has an inlet for the admission of air, the side tube (S), and two exits for its discharge: the annular space (O), and the bore of the jet (J). The first of these exits, the annular space, is a variable aperture, since clockwise rotation of the knob (K) will drive the cone-shaped point of the jet into the annular space, causing the space to close; whilst anticlockwise rotation of the knob will withdraw the jet, causing the space to enlarge. The second exit, the bore of the jet is a fixed aperture. Of the compressed air admitted to the air chamber, some will be discharged from the chamber by way of the bore of the jet valve, and some by way of the annular space; and the way in which the discharge of air is distributed between these two exits determines the pressure in the mount (M). For the air that is discharged from the chamber by the bore of the jet valve is injected into a venturi tube (N), and its entry into the venturi tube will tend to create a negative pressure in the mount; whereas the air that is discharged from the chamber by way of the annular space will enter the mount itself, under pressure—a circumstance that tends to create a positive pressure in the mount. The two tendencies vary in strength as the airflows by which they are produced vary in strength; and since the anaesthetist can, by manipulating the knob (K), increase the strength of either flow, it follows that he can accentuate either tendency, and so produce a positive or a negative pressure in the mount at will. And that he can produce a positive or a negative expiratory pressure—because the pressure produced in the mount will be
THE FAZAKERLEY RESPIRATOR
183
To produce a negative expiratory pressure. The anaesthetist turns the knob (K) clockwise in order to reduce the patency of the annular space and so increase its flow resistance. Once the flow resistance of the space exceeds the flow resistance of the bore of the jet valve, most of the air admitted to the air chamber will leave the chamber by the bore of the valve (i.e. by the easier route), with the result that more air will pass through the venturi tube than will pass into the mount. The dominant tendency will, therefore, be that produced by the venturi tube, and the pressure in the mount will become negative—and increasingly so as the knob continues to be turned. Once the space has been completely closed, no air at all can enter the mount and, the venturi effect being unopposed, the pressure in the mount may fall to as low as minus 10 cm EUO. To produce a positive expiratory pressure. The anaesthetist turns the knob (K) anticlockwise in order to increase the patency of the annular space, and so reduce its flow resistance. Once the flow resistance of the space has fallen below that of the bore of the jet valve, most of the air admitted to the air chamber will leave by way of the annular space. More
To produce atmospheric expiratory pressure. The anaesthetist turns the knob (K) one way or the other until the manometer registers atmospheric pressure during the expiratory phase. He will then in effect have matched the flow resistance of the annular space with that of the bore of the jet valve; and equal volumes of air will be passing into the venturi tube and the mount to produce equal but opposite effects. From the point of view of clinical usage, the E.P.C. unit may be said to function only during the expiratory phase of the cycle. The pressure created in the mount by the interaction of the two forces just described does, however, play a minor role during the inflation phase of the cycle. A negative pressure, for example, then acts as an anti-inflationary force; as is evident by the rise in the peak inflation pressure that occurs when a negative pressure is cancelled, and by the fall in the inflation pressure that occurs when the strength of a negative pressure is increased. Conversely, the provision of a positive expiratory pressure raises, and its cancellation lowers, the peak inflation pressure. These side effects to the use of the E.P.C. unit have one, apparently quite unimportant.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
propagated to the pressure bottle and re- air will therefore pass into the mount than produced in the rebreathing bag during will pass into the venturi tube and the the expiratory phase. During the inflation dominant tendency will be that produced phase, the small pressure produced in the by the expansion of air in the mount. The mount by the tendencies under discussion pressure in the mount accordingly becomes is overwhelmed to insignificance by the positive. Once the space is fully open, little or no air enters the venturi tube and, inflation pressure. How the anaesthetist adjusts the expira- the expansion effect of the air entering the tory pressure in practice is explained mount being unopposed, the pressure in the mount rises to around 5 cm H2O. below.
184
BRITISH JOURNAL OF ANAESTHESIA
capable of supplying up to 40 litres of airoxygen mixture a minute and of delivering the mixture, which contains 30 to 35 per cent oxygen, under a pressure of about 100 mm Hg. With the device in use 100 cubic feet of oxygen will run the respirator for from 6 to 8 hours, according to the MANOMETER; PRESSURE RELEASE VALVE; depth and the frequency of the ventilation provided. AIR-ENTRAINING DEVICE Here it would be as well to point out There remain to be described: the manthat the respirator is subject to the law ometer (fig. 2); the pressure release valve of supply and demand, and that the more (fig. 2 (RV)); and the air entraining work that it is called upon to perform, the device. more energy in the form of air does it The manometer is connected with the need to be provided with. The correct flow breathing circuit. Because of that, and be- is always that which provides satisfactory cause it is a water manometer, it serves not ventilation and the type of inflation phase only as a pressure indicator but as a safety (rapid or slow) that the anaesthetist valve as well, for the height and the depth favours. of the manometer tube will limit the positive and the negative pressures that the DIRECT INFLATION OF LUNGS machine can apply to the lungs. The lengths of the tubes joining the The pressure release valve (RV) con- E.P.C. unit with the I.P.P. unit make the sists of a rubber flap covering a hole in the respirator cumbersome and detract from bag mount (BM). The valve lifts to allow its appearance, but their lengths are a conexcess gas to pass from the breathing venience when it is necessary to ventilate circuit to the bottle, and thence to the a patient with air or with an air-oxygen atmosphere during the expiratory phase. mixture in an emergency. For the respiraIt prevents the rebreathing bag from over- tor to be used in this way, the E.P.C. unit distending during use. is detached from the pressure-bottle and The air-entraining device which is connected to the endotracheal or facemask not illustrated here saves oxygen when connection: and the long tubes enable the respirator is worked from a cylinder. this to be done with the minimum of delay. Designed to be connected with B.O.C. oxygen valve MS.55, or MS.56, by CONCLUSION AND APPARATUS FOR STUDIES OF VENTILATION a short length of stout-walled rubber tubing, and with the respirator by not In conclusion, this respirator was demore than 10 feet of ^-inch bore tubing, signed in the hope that the provision of a the device utilizes the injector principle to negative phase would enable some of the convert a small volume of oxygen at a high complications of positive pressure ventilapressure into a larger volume of air and tion to be avoided, but it is emphasized oxygen at a relatively low pressure. It is that as yet there is only the most tenuous result: that the difference between the maximum and the minimum pressure remains constant for any given setting of the inflation pressure control knob (P) no matter how the expiratory pressure control knob (K) may be set and reset.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
THE FAZAKERLEY RESPIRATOR
185
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
The lefthand bellows (M) are connected with the pressure bottle (P) of the respirator. The righthand bellows (N) are connected with the outlet (S), and with a tube (St) to which is attached a catheter whose tip lies near to the tracheal bifurcation. During the expiratory phase, the bellows (M) are emptied by the suction then present in the pressure bottle, or (if it is not intended to use a negative expiratory pressure) by means of weights placed on top of the bellows. As the bellows (M) empty, so the bellows (N) expand. And as the bellows expand they fill with air drawn from the trachea. During the inflation phase, the positive pressure in the pressure bottle expands the bellows (M) and as they expand they compress the bellows (N), driving the tracheal air contained in the latter towards and into the collecting bag attached at (S). Apparatus for the Continual Withdrawal Nonreturn valves incorporated in the bellows (N) maintain the necessary uniof Samples of the Tracheal Air directional flow. Figure 7 shows the respirator connected with a simple absorption system, in which the volume of the exhaled air may be measured by a meter placed in the expiratory side of the circuit, and in which the fresh anaesthetic gases are admitted at a point that precludes their passing through the meter during the expiratory phase. The rest of the apparatus is concerned with the withdrawal of air from the region of the tracheal bifurcation during, and only during, the expiratory phase, and with the delivery of the air withdrawn to a collecting bag attached at (S). (M) and (N) are two bellows linked in such a manner that the expansion of one bellows will result in the compression of the other. FIG. 7
evidence that the use of negative expiratory pressures is in any way protective. It should also be mentioned that the findings of a recent investigation suggest that there are occasions when the use of negative expiratory pressures is undesirable—during anaesthesia for intrathoracic procedures for example, when their use may lead to collapse of the contralateral lung, to underventilation with retention of CO2, and a dangerous reduction in the uptake of oxygen. In the course of this investigation it became necessary (i) to obtain serial samples of the air present in the trachea during expiration, and (ii) to record the mean ventilating pressure. To this end, two pieces of apparatus suitable for use in conjunction with the respirator were designed. Each is briefly described below.
BRITISH JOURNAL OF ANAESTHESIA
186
Tube leading to airway Open
FIG.
8
height (H) is the height of the water column that the pressure in the airway can sustain continuously: it is therefore the mean ventilating pressure, or a close approximation to that pressure. The water levels alter when the mean The Mean Pressure Manometer pressure alters, but they do not oscillate The following method of measuring the mean ventilating pressure is based on the (to any great extent) in response to fluctuafact that the mean or average pressure in tion of the inflation pressure. This is a system is the effective working pressure because the arrangement allows of the or the pressure continuously at work in displacement of a large volume of water over a period of minutes, but does not that system. allow of the displacement of more than a In figure 8 (A) and (B) are two bottles few cubic centimetres of water during the of large capacity joined together by a second or so that each pressure-phase lasts. syphon tube (T). Each bottle is partly ACKNOWLEDGMENTS filled with water. am glad to have this opportunity of thanking When bottle (A) is connected with the Dr.I H. J. V. Morton for his encouragement and helppatient's airway, the mean pressure will, ful suggestions; thanking Dr. A. B. Christie, Mr. Edwards, and Mr. Kenneth Waddington, for if it is greater than zero, work to displace Ronald the interest they have shown in the use of the respiraand of thanking all the other people who have, water from bottle (A) to bottle (B). At first tor; in one way and another, helped me to press on with the water level in bottle (B) rises steadily; the building of the machine and with the writing of paper. but very shortly it ceases to rise and re- thisI also acknowledge the technical assistance I have mains at a constant height above the received from Messrs Elliotts (Liverpool) Ltd., the manufacturers of the respirator. They could not have water level in bottle (A). The difference in been more co-operative. A tube leading to an infrared gas analyser is substituted for the collecting bag when a breath-by-breath analysis of the exhaled air is required.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on July 1, 2015
Water level (initial)